1. Field of the Invention
The present invention relates to an electromechanical transducer device, such as an ultrasonic transducer, and to a method of making the electromechanical transducer device.
2. Description of the Related Art
Ultrasonic transducers, which convert an electric signal into ultrasound and vice versa, are used as probes for medical imaging and non-destructive testing. There is a type of ultrasonic transducer called a capacitive micromachined ultrasound transducer (CMUT). A CMUT includes, for example, a substrate including a lower electrode, a membrane that is supported by a supporting unit formed on the substrate, and an upper electrode. The lower electrode, the membrane, the upper electrode, and the supporting unit surround a cavity. The CMUT emits a sound wave (oscillatory wave) when a voltage is applied between the lower electrode and the upper electrode and thereby the membrane is vibrated. On the other hand, a CMUT detects a sound wave (oscillatory wave) when the sound wave is received and vibrates the membrane and thereby the capacitance between the lower electrode and the upper electrode is changed.
Traditionally, CMUTs have been made by so-called surface micromachining, bulk micromachining, and the like. Examples of wiring methods include a method of forming an element including a plurality of membranes and cavities on a silicon substrate and connecting the element to a circuit board through the silicon substrate that serves as a lower electrode and a through-hole interconnection (US 2006/0075818 A1). Referring to FIG. 6, this method will be described. A CMUT 1007 includes a plurality of elements 1008, and each of the elements 1008 individually sends and receives ultrasound. Each of the elements 1008 includes an upper electrode 1000, a membrane 1001, a cavity 1002, and a lower electrode 1003. Grooves 1004 are formed so as to electrically isolate and insulate the elements 1008 that are adjacent to each other. The CMUT 1007 is connected to an ASIC substrate 1006 through bumps 1005.
In the existing CMUT, it is necessary to form the separation grooves each having a width of several hundred micrometers in order to electrically isolate the elements from each other, each of the elements including one or a plurality of cavities. Because the mechanical strength of a portion surrounding a cavity is low, it is necessary to support the portion with a supporting member. However, cavities cannot be formed in a region that corresponds to the separation groove (directly above the separation groove) and for which it is difficult to support a cavity. Therefore, the number of cavities that can be formed per unit area of a substrate is limited. As a result, the fill factor (which refers to the proportion of the area occupied by electromechanical transducer elements in the present specification) is reduced. Therefore, the sensitivity of the device is reduced.